In my opinion best cocktail would be 4 talks - total not more than 1hr
30mins like this+1 .
a.LPI/RHCE/Sysadmin talk -20 minutes
b.Programming series talk - 20 minutes
c.Demo of some application like graphic, animation,netmeeting etc. -20some more high level topics :
minutes
d.A talk by student/newbie on what he/she learnt/used recently -20min
e.General disscussion
If we get some highlevel topics like device drivers, cryptography etc.,